libiio: Add a patch from upstream to fix building without a C++ compiler

This fixes a build issue reported by the autobuilder:

   http://autobuild.buildroot.net/results/5b0d02aa3bdb4cddb0d316c99fada2e7ba9f9c1d/



[Thomas: add autobuilder reference, as suggested by Baruch.]

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>